Rhodium is an active metal component used in three-way catalyst systems for the reduction of oxides of nitrogen to nitrogen gas. Rhodium is selective in the reduction of oxides of nitrogen by carbon monoxide in the presence of excess oxygen. In a condition where there is not an excess of oxygen, rhodium reduces oxides of nitrogen to nitrogen gas with little formation of ammonia gas.
Rhodium is a by-product of the mining of platinum. On a world-wide basis, normally about 17 units of platinum are mined for each unit of rhodium. Therefore, the rhodium content of any catalyst system must be used effectively and its effectiveness must be maintained during the operative life of the catalyst system.
As reported in the Journal of Catalysis, Volume 50, pages 407 to 418 (December, 1977), in an article entitled "Surface Interaction in the System Rh/Al.sub.2 O.sub.3 ", rhodium interacts strongly with gamma alumina, which is normally used as a wash coat material for catalyst systems. Under oxidizing conditions at elevated temperatures, rhodium diffuses into the bulk of the gamma alumina. This diffused and dissolved rhodium is only partly recovered when reducing conditions are once again established over the catalyst system. Thus, exposure of a rhodium containing catalyst system based upon a gamma alumina wash coat to high temperature conditions results in the loss of rhodium as an effective catalyst material over the life of the catalyst system.
It is a particular object of this invention to provide a catalyst system in which rhodium may be employed as a catalyst and the effectiveness of that rhodium is maintained over the life of the catalyst system.